[codex] Add workspace diff viewer plugin (#6071)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies.
> - Operators need to inspect what agents changed inside execution and
project workspaces.
> - The existing workspace detail views did not provide a first-party
rich diff surface for staged, unstaged, head, renamed, binary,
oversized, and untracked changes.
> - The plugin system is the intended extension point for optional rich
UI surfaces.
> - This pull request adds a workspace diff plugin plus host services
and shared contracts so Changes tabs can render workspace diffs through
plugin slots.
> - The diff-renderer dependency should stay owned by the plugin package
rather than the core UI app.
> - The dependency surface must stay aligned with repository PR policy,
including intentionally omitting `pnpm-lock.yaml` from the PR.
> - The benefit is a more reviewable workspace surface without
hard-coding the renderer into every page.

## What Changed

- Added `@paperclipai/plugin-workspace-diff`, including diff
normalization, plugin manifest/worker/UI entrypoints, and focused plugin
tests.
- Kept `@pierre/diffs` scoped to `@paperclipai/plugin-workspace-diff`;
removed the core UI lab diff-renderer surface and direct UI package
dependency.
- Added shared workspace diff types and validators, plus plugin SDK
surface for workspace diff host services.
- Added server workspace diff service support and route coverage for
execution/project workspace diff flows.
- Wired Execution Workspace and Project Workspace Changes tabs to load
the diff plugin, including loading/error fallback behavior.
- Added UI tests and fixtures for the Changes tabs and plugin bridge
behavior.
- Added the new plugin package manifest to the Docker deps stage so PR
policy can validate dependency coverage.
- Addressed review hardening around empty untracked patches, workspace
path exposure, project workspace read capability checks, and default
base refs.

## Verification

- `pnpm --filter @paperclipai/plugin-workspace-diff test`
- `pnpm exec vitest run
packages/shared/src/validators/workspace-diff.test.ts
server/src/__tests__/workspace-diff-service.test.ts
ui/src/pages/ProjectWorkspaceDetail.test.tsx
ui/src/pages/ExecutionWorkspaceDetail.test.tsx`
- `pnpm exec vitest run ui/src/plugins/bridge.test.ts
server/src/__tests__/workspace-runtime-routes-authz.test.ts`
- `pnpm --filter @paperclipai/shared typecheck`
- `pnpm --filter @paperclipai/plugin-workspace-diff typecheck`
- `pnpm --filter @paperclipai/server typecheck`
- `pnpm --filter @paperclipai/ui typecheck`
- `node ./scripts/check-docker-deps-stage.mjs`
- Browser screenshot captured from the local worktree dev server:
https://files.catbox.moe/ofdpsp.png
- Confirmed branch is rebased onto `public-gh/master`,
`.github/workflows/pr.yml` is not included in the PR diff,
`ui/package.json` is not included in the PR diff, and `pnpm-lock.yaml`
is not included in the PR diff.

## Risks

- Medium UI integration risk: the Changes tab depends on the plugin slot
and host diff service path.
- Medium dependency risk: this adds `@pierre/diffs` in the plugin
package, but `pnpm-lock.yaml` is intentionally omitted per packaging
instructions because repository automation manages lockfile updates.
- Current CI blocker: downstream frozen installs fail until the
repository policy path for new plugin package dependencies is chosen.
- Diff rendering edge cases are covered for common working-tree and head
diff states, but very large repositories may still expose performance
limits.
- No migrations are included.

> For core feature work, check [`ROADMAP.md`](ROADMAP.md) first and
discuss it in `#dev` before opening the PR. Feature PRs that overlap
with planned core work may need to be redirected — check the roadmap
first. See `CONTRIBUTING.md`.

## Model Used

- OpenAI Codex, GPT-5 class coding model, tool-enabled local execution
environment. Exact context window was not exposed by the runtime.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] If this change affects the UI, I have included before/after
screenshots
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>

packages/plugins/sdk/src/testing.ts

@@ -33,6 +33,7 @@ import type {
   ToolResult,
   ToolRunContext,
   PluginWorkspace,
+  PluginExecutionWorkspaceMetadata,
   AgentSession,
   AgentSessionEvent,
   PluginLocalFolderEntry,
@@ -80,6 +81,8 @@ export interface TestHarness {
     issueComments?: IssueComment[];
     agents?: Agent[];
     goals?: Goal[];
+    projectWorkspaces?: PluginWorkspace[];
+    executionWorkspaces?: PluginExecutionWorkspaceMetadata[];
   }): void;
   setConfig(config: Record<string, unknown>): void;
   /** Dispatch a host or plugin event to registered handlers. */
@@ -438,6 +441,7 @@ export function createTestHarness(options: TestHarnessOptions): TestHarness {
   const agents = new Map<string, Agent>();
   const goals = new Map<string, Goal>();
   const projectWorkspaces = new Map<string, PluginWorkspace[]>();
+  const executionWorkspaces = new Map<string, PluginExecutionWorkspaceMetadata>();
   const localFolderStatuses = new Map<string, PluginLocalFolderStatus>();
   const localFolderFiles = new Map<string, string>();
 
@@ -975,6 +979,13 @@ export function createTestHarness(options: TestHarnessOptions): TestHarness {
         },
       },
     },
+    executionWorkspaces: {
+      async get(workspaceId, companyId) {
+        requireCapability(manifest, capabilitySet, "execution.workspaces.read");
+        const workspace = executionWorkspaces.get(workspaceId);
+        return workspace?.companyId === companyId ? workspace : null;
+      },
+    },
     routines: {
       managed: {
         async get(routineKey, companyId) {
@@ -2048,6 +2059,12 @@ export function createTestHarness(options: TestHarnessOptions): TestHarness {
       }
       for (const row of input.agents ?? []) agents.set(row.id, row);
       for (const row of input.goals ?? []) goals.set(row.id, row);
+      for (const row of input.projectWorkspaces ?? []) {
+        const list = projectWorkspaces.get(row.projectId) ?? [];
+        list.push(row);
+        projectWorkspaces.set(row.projectId, list);
+      }
+      for (const row of input.executionWorkspaces ?? []) executionWorkspaces.set(row.id, row);
     },
     setConfig(config) {
       currentConfig = { ...config };